The performance of synthetic aperture radar (SAR) image interpretation directly depends on the image quality. However, conventional SAR image-quality indicators are measured to check whether the SAR system has maintained its performance specifications, not to assess how well the SAR image can serve image interpretation. An SAR image-quality assessment method based on the modulation transfer function (MTF) is proposed for image interpretation, which jointly reflects the resolution and the contrast of the SAR image. In addition, it describes the imaging performance of the SAR system at different spatial frequencies. Specifically, we propose a feasible MTF test field and realize it in Shanghai Jiao Tong University, Shanghai, China. Then, we give the MTF measurement procedure and utilize the MTF curve to evaluate SAR image quality. Simulation results demonstrate that the proposed MTF-based method is more accurate to assess the SAR image quality than the conventional methods. In addition, the spaceborne SAR experiments are carried out by the TerraSAR-X sensor and the experimental results are given to confirm the benefits of the proposed method.